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The Veterinary Journal xxx (2009) xxx–xxx
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The Veterinary Journal
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Comparison of pressure plate and force plate gait kinetics in sound Warmbloods
at walk and trot
Maarten Oosterlinck a,*, Frederik Pille a, Tsjester Huppes b, Frank Gasthuys a, Willem Back a,b
a
b
Department of Surgery and Anaesthesiology, Faculty of Veterinary Medicine, Ghent University, B-9820 Merelbeke, Belgium
Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, NL-3584 CM Utrecht, The Netherlands
a r t i c l e
i n f o
Article history:
Accepted 18 August 2009
Available online xxxx
Keywords:
Horse
Force plate
Gait analysis
Pressure plate
Symmetry
a b s t r a c t
Modern pressure plates (PP) could be an alternative to traditional force plates (FP) for quantitative equine
gait analysis, thereby providing the clinician with objective data on the horse’s gait while unravelling the
loading of different regions of the hoof during the stance phase. The aim of this study was to determine
whether a stand-alone PP allows reliable measurement of gait kinetics, compared to simultaneously
recorded FP variables. Six sound Warmblood horses were walked and trotted over a combined PP and
FP system for collection of a set of five valid kinetic measurements for each forelimb. A measurement
was considered valid if the horse was moving in a straight line at a constant pace while gait velocity
was within a preset range and the hoof fully contacted the plate surface.
Significant differences between FP and PP data were seen for peak vertical force (PVF), vertical impulse
(VI), time at which the PVF occurs (tPVF) and forelimb symmetry ratios (SymPVF and SymVI) (P < 0.05),
but not for stance phase duration (ST). Nevertheless, mean agreement indices (AIs) of ST, tPVF and SymPVF and SymVI were excellent (P0.92), whereas AIs of PVF and VI were moderate (P0.70). The excellent
agreement between PP and FP symmetry ratios confirms that observed differences between PP and FP in
symmetry ratios are small (2–7%), especially when compared to the expected decrease in symmetry associated with mild lameness (>20%). The results indicate that a stand-alone pressure plate can be used to
measure absolute (ST) and relative (tPVF) temporal variables and loading symmetry ratios and offers
equine veterinarians a mobile, cost-efficient and quick gait evaluation method for routine clinical use.
However, the system cannot be used interchangeably with a force plate to measure absolute values of
limb loading.
Ó 2009 Elsevier Ltd. All rights reserved.
Introduction
Force plates (FPs) are accepted kinetic instruments for the
quantitative evaluation of equine lameness (Buchner, 2001; Bertone, 2003; Ishihara et al., 2009). Despite their recognised accuracy, the use of FP systems is usually restricted to gait analysis
under laboratory environments as installation procedures are laborious and costly as the FP requires a massive concrete foundation,
while concurrent hoof strikes cannot be separated and limited
plate dimensions preclude recording consecutive strides (Buchner,
2001; Bertone, 2003).
Modern portable pressure plate (PP) systems open new possibilities, whereby additional information on the pressure distribution of the different regions of the hoof during a complete stance
period can be obtained (Van der Tol et al., 2003; Van Heel et al.,
2004). This may allow us to differentiate toe and heel landing in
specific causes of lameness (e.g. navicular syndrome vs. laminitis).
* Corresponding author. Tel.: +32 9 264 76 18; fax: +32 264 77 94.
E-mail address: [email protected] (M. Oosterlinck).
PP measurements can be dynamically calibrated using simultaneous FP measurements (Van der Tol et al., 2003; Van Heel et al.,
2004; De Cock et al., 2005; Hagman et al., 2008). However, the
combination of a built-in and fixed FP and PP is really only feasibly
in an institute or laboratory and is not practical use in a cost-efficient, mobile clinical environment. Solitary PPs, operational without concurrent FP calibration, have been applied in horses
(Rogers and Back, 2007; Oosterlinck et al., 2009) and humans (Thijs
et al., 2007). However, before kinetic data obtained with a standalone PP can be used clinically, validation of the system for precision (i.e. agreement between repeated measurements) and accuracy (i.e. agreement with reference standard) is necessary
(Bossuyt et al., 2003).
A high level of precision of gait kinetics and loading symmetry
ratios has been demonstrated in ponies using a stand-alone PP
(Oosterlinck et al., 2009). Studies specifically dealing with the
accuracy of a pressure measuring system compared to a FP have
been undertaken in humans (Chesnin et al., 2000) and dogs
(Besançon et al., 2003). The former reported excellent correlation
between the centre of pressure determined by an in-shoe pressure
1090-0233/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tvjl.2009.08.024
Please cite this article in press as: Oosterlinck, M., et al. Comparison of pressure plate and force plate gait kinetics in sound Warmbloods at walk and trot.
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M. Oosterlinck et al. / The Veterinary Journal xxx (2009) xxx–xxx
measuring system and a FP, while the latter reported statistically
significant differences in gait kinetics between the FP and PP systems. Equine bodyweight (BW), gait variables (stride length, etc.)
and conformation are very different compared to other animal species and humans, necessitating direct comparison of simultaneously recorded PP and FP measurements.
A practical application can be foreseen when variables determined from the stand-alone PP could be used interchangeably with
FP measurements, thereby providing valuable and objective information on the horse’s gait and symmetry. This would offer the
equine clinician a cost-efficient, quick and even portable gait evaluation method for routine clinical use. The objective of the present
study was to compare gait kinetics in a group of sound horses using
simultaneous measurements with a PP and a FP. The hypothesis
was tested that the kinetic gait variables of the PP and the FP can
be used interchangeably.
Materials and methods
Horses
Six healthy and clinically sound Dutch Warmblood mares were included in the
study (mean age 7 ± 4 SD years, BW 592 ± 82 kg; wither height 1.59 ± 0.07 m). All
horses were part of the teaching herd of Utrecht University, and were regularly used
for pleasure riding by the student riding school. Prior to the study, each horse was
physically evaluated by a veterinarian and judged to be healthy and sound.
The study was approved by the Ethical Committee of Utrecht University
(2008.III.07.071).
Measurement systems and data collection
After a 5 min warm-up period, the horses were led over the measuring system
by one experienced handler, first at the walk and subsequently at trot. The measuring system consisted of the PP (Footscan 3D 1 m-system, RsScan International)
mounted on top of the force platform (Z4852C, Kistler) (Fig. 1), embedded flush
and in the middle of a 20 m long track, covered with a 5 mm rubber mat. This setting has been used in equine (Van Heel et al., 2004), human (De Cock et al., 2005;
Hagman et al., 2008) and bovine studies (Van der Tol et al., 2003). FP and PP data
were collected simultaneously and recorded on separate notebook computers (Precision M6400, Dell) with dedicated software (Bioware 4, Kistler; Footscan Scientific
Gait 7, RsScan International).
Calibration of the FP was verified before and shortly after the entire study. At
the start of the study, the PP was calibrated in accordance with manufacturer’s
specifications using the RsScan software (Footscan Scientific Gait 7, RsScan International) with a person weighing 80 kg. In an attempt to further optimise the calibration procedure, the recalibration factor obtained in this way was fine-tuned using
one of the horses in the study. The horse was weighed with one forelimb standing
on a calibrated weighing scale that was embedded flush in a concrete floor, standing
squarely, while the contra-lateral forelimb was held in a flexed position without
supporting weight. After the horse was placed on the PP in the same position, the
constant vertical force recorded at that moment was compared with the previously
recorded weight. The recalibration factor was changed manually until these two
values matched for five consecutive calibration trials (i.e. a maximal difference of
5% BW).
The FP and the PP were both set at their routinely used measuring frequency
(1000 Hz and 250 Hz, respectively) so that for each frame of the PP, four FP frames
were recorded. Both plates were triggered simultaneously by a photocell-activated
gate 0.20 m in front of the measuring surface.
The average velocity over the measuring area was computed using two pairs of
photoelectric-sensors (WE260-S270, Sick) that were placed as two gates perpendicular to the runway at 2 m interval centred over the measuring area, connected to an
electronic timing box (Timer Interval Meter K3HB-P, Omron).
Although acceleration was not measured, the 20 m long track ensured that the
effect of acceleration and deceleration at the start and end of each trial was minimised over the central 2 m measuring area. A trial was considered valid if the horse
moved at a constant pace, looking straight forward, while gait velocity was within a
preset range of 0.9–1.5 m/s at the walk and 1.8–3.5 m/s at the trot and the hoof of at
least one forelimb fully contacted the plate surface. A total number of five valid
measurements were collected for both forelimbs.
For both forelimbs of all horses, the following kinetic variables were calculated:
(1) peak vertical force (PVF) was normalised by horses’ bodyweight and expressed
as Newtons per kilogram (N/kg); (2) vertical impulse (VI) was calculated by time
integration of the force–time curves and multiplied by time, normalised by weight
and expressed as Newton-seconds per kilogram (N s/kg); (3) stance time (ST) was
calculated as the difference between the end and the beginning of the stance phase,
and was expressed in milliseconds (ms), and (4) time at which the PVF occurs (tPVF)
was expressed as a percentile of the duration of stance time (%stance).
Data processing
Only the data from the five valid trials at walk and at trot were analysed. For
each set of five trials, the mean left fore (LF) and right fore (RF) value were calculated as being representative for that limb of each animal. Forelimb symmetry
ratios (SymPVF and SymVI; %) were calculated as lowest/highest mean
value 100%, as described by Merkens et al. (1986).
Statistical analysis
Data were collated and prepared for statistical analysis using spreadsheet software (Microsoft Office Excel 2003) and statistical analysis was performed using
SPSS 16.0 with statistical significance set at P < 0.05. Data are presented as
means ± SD, unless otherwise stated.
The assumption of normal distribution of all variables was confirmed using rankits plot, Kolmogorov–Smirnov and Shapiro–Wilk test. A linear Mixed Effects Model
with horse as random component was used to evaluate (1) the effect of gait, LF or RF
and measuring system on PVF, VI, ST and tPVF and (2) the influence of gait and measuring system on PVF and VI symmetry ratios.
To examine the agreement between PP and FP data, mean ± SD agreement indices (AI) were calculated for all variables. The following AI equation was used (Van
der Vlugt-Meijer et al., 2006; White et al., 2008):
AI ¼ 1 ½jPP FPj=fðPP þ FPÞ=2g
1
2
3
7
4
6
5
5
Fig. 1. Transverse section of the combined force plate and pressure plate system embedded flush in the runway. 1: pressure plate; 2: aluminium plate interspaced between
pressure plate and force plate; 3: force plate; 4: steel mounting frame of the force plate; 5: concrete foundation; 6: drain; 7: cable channel.
Please cite this article in press as: Oosterlinck, M., et al. Comparison of pressure plate and force plate gait kinetics in sound Warmbloods at walk and trot.
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M. Oosterlinck et al. / The Veterinary Journal xxx (2009) xxx–xxx
An AI of 1 indicated perfect agreement and mean AIs P 0.90 represented excellent
agreement (Van der Vlugt-Meijer et al., 2006; White et al., 2008), while
(0.7 6 AI < 0.8) was considered moderate and (0.8 6 AI < 0.9) good agreement.
Table 1
Means ± SD of concurrent pressure plate (PP, index test) and force plate (FP, reference
standard) measurements at walk and trot in six horses with their agreement indices
(AI).
Results
A representative example of vertical force curves at walk and at
trot, obtained with both measuring systems, is shown in Fig. 2. PP
and FP measurements at walk and trot of all horses (n = 6), and AIs
are summarised in Table 1.
Except for the symmetry ratios, analyses were conducted on
pooled LF and RF data of the six horses since no significant differences were found between both forelimbs. All variables showed
significant differences between the walk and trot. PVF, VI and tPVF
obtained with the PP system were significantly different from the
ones obtained from the FP, whereas only ST was not significantly
different between the two measuring systems (Table 1). At walk
and trot, SymPVF and SymVI of both systems were around 90%,
but the PP SymPVF and SymVI were nevertheless significantly lower than the FP values.
A significant interaction between gait and measuring system
was seen for PVF and tPVF, but not for the other variables. Pairwise
comparisons (Scheffé test) of combinations of interacting factors as
separate groups (walk-PP, walk-FP, trot-PP and trot-FP) showed
significant differences in all of these groups: at walk PP PVF was
on average 1.4 N/kg lower than FP PVF, while at the trot the mean
difference was 2.7 N/kg. At walk, PP tPVF occurred on average 3.8%
a
Variables
PP
FP
AI
Walk
PVF (N/kg)
VI (Ns/kg)
tPVF (%stance)
ST (ms)
SymPVF (%)
SymVI (%)
5.1 ± 1.1a
2.9 ± 0.7a
65.6 ± 3.9a
834 ± 38
93.7 ± 3.2a
88.7 ± 7.3a
6.5 ± 0.1b
3.6 ± 0.2b
61.8 ± 3.5b
834 ± 39
98.2 ± 0.9b
96.0 ± 2.0b
0.74 ± 0.23
0.75 ± 0.24
0.99 ± 0.01c
1.00 ± 0.00c
0.95 ± 0.04c
0.92 ± 0.06c
Trot
PVF (N/kg)
VI (Ns/kg)
tPVF (%stance)
ST (ms)
SymPVF (%)
SymVI (%)
7.8 ± 1.5a
1.5 ± 0.4a
51.9 ± 4.6a
332 ± 23
92.0 ± 3.3a
92.0 ± 4.0a
10.4 ± 0.7b
2.0 ± 0.2b
43.0 ± 2.9b
333 ± 23
97.5 ± 2.9b
94.8 ± 2.4b
0.69 ± 0.19
0.72 ± 0.20
0.95 ± 0.01c
0.98 ± 0.01c
0.94 ± 0.03c
0.97 ± 0.05c
PVF, peak vertical force; tPVF, the time at which the PVF occurs; ST, stance phase
duration; VI, vertical impulse; SymPVF and SymVI, forelimb symmetry ratios.
a,b
Within the same row, values with different lowercase superscript letters differ
significantly (P < 0.05).
c
Mean AI P0.90 representing excellent agreement.
later in stance than in the FP measurements, while at trot it was on
average 8.9% later.
Irrespective of gait, mean AIs for ST, tPVF, SymPVF and SymVI
were P0.92 with SD 6 0.06, while AIs for PVF and VI at both gaits
were P0.70 with SD 6 0.24 (Table 1).
8
Vertical Force (N/kg)
7
6
5
4
3
2
1
0
0
10
20
30
40
50
60
70
80
90
100
70
80
90
100
Time (%stance time)
b
12
Vertical Force (N/kg)
10
8
6
4
2
0
0
10
20
30
40
50
60
Time (%stance time)
Fig. 2. Representative example (n = 1) of a vertical ground reaction force (N/kg) curve as a function of time (%stance time) recorded simultaneously with a pressure plate
(dashed line) and a force plate (solid line) at walk (a) and trot (b).
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Discussion
The vertical force curves at walk and trot, obtained with both
measuring systems, showed a similar pattern as reported earlier
by Oosterlinck et al. (2009). Nevertheless, in the present study
the actual PVF recorded with the PP showed a lower maximal value
and was recorded somewhat later in the stance phase compared to
the FP measurement.
Absolute stance duration recorded with PP and FP was similar
and excellent agreement between both measuring systems was
identified for relative temporal variables (tPVF). However, these
temporal variables may not be useful kinetic parameters with
which to predict equine gait anomalies, due to the rather large inter-individual variability and low sensitivity for detection of mild
lameness (Ishihara et al., 2005). The observed differences in PVF
and VI symmetry ratios (2–7%; Table 1) were relatively small,
whilst excellent agreement between PP and FP symmetry ratios
was identified. Only moderate agreement was seen for absolute
PVF and VI values.
A number of causes may explain the differences in data collected from the PP and the FP. There may be possible latency,
and the temporal resolution of the sensors of the PP. PP PVF and
VI were consistently lower than FP values. In contrast, a higher
tPVF value was found for the PP compared to the FP whereas stance
duration was highly similar between both systems. These findings
might indicate a slower response (i.e. latency) of the multitude of
resistor-based sensors of the PP compared to the four piezoelectric
quartz crystal sensors of the FP. In the lower range of measurements, i.e. at the beginning and end of stance, the PP showed excellent alignment with the FP. Hagman et al. (2008) found no delays
and no significant inaccuracies in the internal clocks of both systems and concluded synchronisation of the beginning and end of
stance between both systems was excellent. However in the upper
range of the forces in the present study, i.e. around PVF, the PP sensors showed a slower response than the FP. Moreover, at the trot
PVF and tPVF showed larger differences than at walk thereby confirming larger deviations with increased loading.
The lower measuring frequency of the PP (250 Hz) compared to
the FP (1000 Hz) might also have contributed to the differences,
although this would be likely to change the smoothness of the
force–time curves rather than diminish the height of the curves.
An illustration of this lower temporal resolution is the finding that
the oscillations in the vertical force curve recorded with the FP at
the beginning of the stance phase were smoothed in the PP curve
(Fig. 2). The lower frequency of the PP may lead to missed data
points and therefore a less accurate reading of the PVF, as suggested by Besançon et al. (2003) using another pressure measurement system in dogs. Nevertheless this is unlikely to be a major
factor for the observed differences between PP and FP, as measuring frequency of the PP (250 Hz) was considered sufficiently high.
A further reason for differences between both measuring systems may be a suboptimal calibration of the PP. To the authors’
knowledge, extensive data are lacking in the literature. Compared
to the standard procedures specified by the manufacturer, Oosterlinck et al. (2009) used a refined calibration technique as described
by Putti et al. (2008) whereby a static measurement was performed with a person standing on the PP on one limb. Subsequently, the recalibration factor was fine-tuned until the
recorded vertical force was equivalent to the bodyweight of that
person. As the linear response of the pressure-sensitive sensors
may deviate under the heavy load of the adult Warmblood horses
used in our study, it might be recommended to perform calibration
in the same range as the actual measurements.
A static heavy weight in the range of a horse’s BW is not readily
available. Therefore our calibration procedure was refined using a
horse standing with one forelimb on the plate. This can easily be
performed in a practical setting. In the laboratory, PP measurements can be dynamically calibrated with a FP, as has been reported with humans (De Cock et al., 2005), horses (Van Heel
et al., 2004) and cattle (Van der Tol et al., 2003). The presence of
the PP on top of the FP does not change the vertical force signal
in humans (Hagman et al., 2008), implying that continuous dynamic calibration of a PP using the vertical force signal of a FP is
possible. However, this dynamic calibration was not done in the
present study as we wanted to evaluate the clinical applicability
of this portable PP as a stand-alone system.
The velocity range reported here (0.9–1.5 m/s at walk and 1.8–
3.5 m/s at trot) was wider than that found in our previous study
(1.1–1.5 and 2.5–3.5 m/s, respectively) to enhance simplicity of
data collection. This undoubtedly caused larger inter-trial variability whereas in the previous work excellent repeatability of kinetic
symmetry ratios was shown (Oosterlinck et al., 2009). However, in
the present study variability was equivalent for the PP and the FP,
as measurements with both systems were performed simultaneously. Although not considered in the present study, variability
of gait kinetics can be a parameter of interest, as Ishihara et al.
(2009) have shown that a unilateral decreased PVF and increased
variability of gait kinetics may be considered as pathognomonic
of musculoskeletal gait abnormality in horses.
Back et al. (2007, 2009) reported that visually scored grade 2/5
lameness in Dutch Warmbloods is associated with a reduction of
the absolute PVF from 118% to 89% BW, compatible with a decrease
in symmetry around 20–25% or more, as compensatory overloading of the sound limb may occur. Therefore, the clinical relevance
of the differences between the PVF and VI symmetry ratios in the
present study appeared low, and the accuracy of the PP should
be considered clinically acceptable for these symmetry ratios. This
is in agreement with Besançon et al. (2003), who reported statistically significant differences in PVF and VI data between a FP and a
pressure measuring system in dogs, but stated that the clinical relevance appeared negligible, indicating the pressure-measuring
walkway was a viable alternative to the FP.
Although in the present study differences in symmetry ratios
were observed between both measuring systems, PP kinetic symmetry ratios are highly repeatable (Oosterlinck et al., 2009) and this
is critical in the objective evaluation of lameness resolution after local analgesia and longitudinal evaluation of orthopaedic treatments.
Additionally, the use of a larger PP allows recording consecutive
hoof strikes in one passage, at least in ponies (Oosterlinck et al.,
2009). This is a powerful tool, as contralateral limbs are evaluated
in one and the same passage, excluding inter-trial variability in
the calculation of left–right symmetry. Besides these distinct advantages of this PP prospecting repeated gait assessments, pressure
measuring equipment with high spatial and temporal resolution allows detailed evaluation of the pressure distribution within each
hoof (Van der Tol et al., 2003; Van Heel et al., 2004).
Conclusions
We are aware that this stand-alone PP cannot simply replace a
FP when high accuracy of absolute force values is needed for scientific purposes. Extended calibration may help to reduce measuring
errors. Nevertheless, the available PP system can be used standalone to evaluate temporal variables and kinetic symmetry ratios
in horses. Moreover, with PP measurements, the pressure distribution of the different regions of the hoof during a complete stance
phase might become unravelled (Van der Tol et al., 2003; Van Heel
et al., 2004), and analysis of simultaneous hoof impacts can be performed, which are not possible using a FP analysis. With these
Please cite this article in press as: Oosterlinck, M., et al. Comparison of pressure plate and force plate gait kinetics in sound Warmbloods at walk and trot.
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advantages, it is tempting to explore the use of the stand-alone PP
to quantify equine lameness in a clinical setting. The present study
opens perspectives to evaluate the use of the PP in sound and lame
horses in order to demonstrate the utility of this pressure measuring system for repeated gait assessment in the horse (e.g. before
and after application of diagnostic analgesia or corrective shoeing
techniques).
Conflict of interest statement
None of the authors of this paper has a financial or personal
relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Acknowledgements
The authors wish to thank Dr. Hans Vernooij (Utrecht University) and Professor Jeroen Dewulf (Ghent University) for their assistance with statistical analyses, and Dr. Meike Van Heel (Mustad
Hoofcare) for critically reading the manuscript.
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Please cite this article in press as: Oosterlinck, M., et al. Comparison of pressure plate and force plate gait kinetics in sound Warmbloods at walk and trot.
The Veterinary Journal (2009), doi:10.1016/j.tvjl.2009.08.024